To make this shell soup gluten-free, simply substitute the regular shell pasta with a gluten-free pasta alternative, such as chickpea, rice, or lentil shells. Additionally, ensure that the beef bouillon cubes used are certified gluten-free, as some varieties contain wheat-based additives. Adjust cooking time accordingly, as gluten-free pasta tends to cook more quickly and can become overly soft if left to simmer for too long.